1. Field of the Invention
The present invention is directed to a medical implant having at least a part thereof which comes into contact with body tissue and/or body fluids.
2. Description of the Prior Art
Piezoelectric materials have a widespread use in the medical field. The materials have for instance found use in electrodes and sensors for implantation in the body of a living being.
The sensors may for instance be accelerometers measuring the general movements of the body or the movements of the heart or parts thereof. The sensors may also be pressure sensors or sensors based on emitting/receiving ultrasound. The electrodes may be electrodes for mechanical/electric stimulation.
The piezoelectric materials used in the medical field for sensing purposes or for mechanical stimulation must meet high standards in regard of for instance sensitivity and durability. One consequence of this is that many known piezoelectric materials are less suitable for this purpose.
The commercially available biocompatible sensor materials have the disadvantage that sensors made of these materials consume too much current. Their mechanical and chemical strength is poor and is not sufficient for a lifetime of for instance 10 years. The biological stability also is poor since clotting and overgrowth reduces the efficiency of the sensors in view of their inherent softness. In spite of blood attenuating additives in the sensitive membrane, the best commercial sensors have a maximum lifetime of only six months.
Biocompatible phosphate glass ceramics that may contain crystal phases of apatite and AlPO4 in the tridymite and/or berlinite form are disclosed in U.S. Pat. No. 4,698,318. Berlinite is an isotype to quartz and has inherent piezoelectric properties. It is suggested that the piezoelectric properties of berlinite can be utilized to promote healing of bone fractures. Berlinite has relatively weak piezoelectric properties. Since the berlinite forms only a part of the material, the overall piezoelectric properties of this material are weak. The piezoelectric properties are obtained by thermal treatments at relatively high temperatures for long time periods, said to cause targeted precipitation of apatite or of apatite and AlPO4-crystals. The long-term stability of the material in the implanted state is not discussed, but hydroxylapatite and apatite are at least to some extent biodegradable.
The piezoelectric materials meeting the desirable, high standards that are necessary or desirable for implants normally contain components that pose a risk of being harmful in the implanted state, for instance lead, as for instance in lead titanate, lead metaniobate or lead zirconate, and it thus is undesirable that these materials directly or indirectly come into contact with body tissue or fluids. To eliminate such risks, however small, it is advisable to carefully enclose the piezoelectric material in an inert, stable enclosure meeting very high standards as to longevity. For instance, sensors normally are left in the body for a very long time and often are not removed at all, even in the event of failure. One example of this is pacer electrodes/sensors for implantation in the heart.
Apart from complicating the design of the implants as well as increasing the cost thereof, the enclosures may also adversely affect the sensitivity of the piezo structure, since the mechanical forces involved in the function of the piezo material must be transferred through at least one wall of the enclosure.
An object of the invention is to provide a piezoelectric medical implant that has a sensitivity and a durability that meets the high standards required and which further is biocompatible, i.e. which may be allowed to come into contact with body tissue or fluids.
The above object is achieved in accordance with the present invention in a medical implant having an element with at least a part thereof composed of piezoelectric material, the piezoelectric material comprising NaxKyNbO3, with 0xe2x89xa6xxe2x89xa60.8, 0.2xe2x89xa6yxe2x89xa61, and x+y=1.
It has surprisingly been found that the above-described piezomaterials have a biocompatibility comparable to that of titanium.